1. Field of the Invention
The invention relates generally to x-ray microanalysis and more specifically to advanced performance energy dispersive lithium-drifted silicon x-ray detectors in spectrometry.
2. Description of the Prior Art
Low noise and consequent high resolution capabilities of lithium-drifted silicon (Si(Li)) x-ray spectrometers are achieved by operating a semiconductor detector and associated junction field effect transistor (JFET) at temperatures approximately that of liquid nitrogen (LN). Operation at low temperatures, typically -140.degree. C., reduces the noise associated with thermally generated charge carriers in the semiconductor detector and JFET preamplifier. (See, Madden, et al., "A High Resolution Si(Li) Spectrometer with Thermoelectric Cooling," prepared for the U.S. Department of Energy under contract W-7405-ENG-48, and submitted to Nuclear Instruments and Methods, July 1978). In an early effort a detector was operated at temperatures in the range of -40.degree. C. to -50.degree. C. and a resolution of 1.6 keV full width at half maximum (FWHM) was attained. (Id., citing E. Belcarz et al., Nukleonika 19 (1974) 1043; Translation: Nukleonika 19 (1974) 13.)
A major disadvantage of cooling with LN is the necessity for a large, bulky reservoir, or dewar. Using LN is hazardous, inconvenient, and costly. Serious burns can result from skin contact with LN. The LN dewars must be constantly replenished in order to maintain the silicon crystal detector at operating temperatures. Typically, this must be done twice per week. X-ray detectors are usually mounted beyond easy reach, and refilling the LN dewar is often a dangerous operation. Users must carefully follow safety procedures or risk injury. The costs of purchasing, storing and handling liquid nitrogen over the lifetime of an x-ray detector can be very high. LN evaporates from both the detector dewar and from the storage tank. Also, during the refilling process, significant amounts of LN are lost due to evaporation and spillage. Thus, in addition to the expense of LN consumed by the detector itself, one must also add budget for LN lost to the atmosphere during dewar refilling and storage. About ten liters of LN per week evaporate from a standard detector dewar, and about the same amount is lost during transfer and evaporation from the storage tank. Therefore, a standard 160-liter storage tank lasts about eight weeks and must be refilled an average of 6.5 times per year. Annual costs of operating an LN-based detector are approximately $1,500 per year.
To the annual cost of LN must be added the labor for ordering and transferring LN, and refilling the detector dewar. Even if this labor averages only twenty minutes per week, it can add hundreds of dollars per year to the cost of servicing an LN-cooled x-ray detector. In addition, the handling of LN often requires that special liability insurance be purchased.
Prior art LN-cooled x-ray detectors are bulky, due to their (typical) nine liter dewars, and weigh over forty pounds. Placed on the end of a long support structure, an LN-cooled detector is cantilevered, and is prone to vibrate and interfere with high resolution imaging in its application on electron microscopes.
Some limited commercial use of Peltier cooled Si(Li) detectors in spectrometers is present in the prior art. Such detectors were generally limited to x-ray diffraction goniometer use. Madden, et al, report some usefulness in energy-dispersive x-ray fluorescence analysis, although with resolution less than that achievable with LN cooling. (Madden, et al., supra.) The short snoots of the prior art prohibited their use in high resolution SEM's. The long snoots needed in SEM applications of x-ray detectors were not practical, because the lever arm of a longer snoot would twist the Peltier cooling device and crack it, and also because fan motor vibrations would be amplified by the long snoot and would degrade SEM performance. In 1985, Kevex Instruments (San Carlos, CA) introduced a Peltier cooled energy dispersive x-ray detector. Called the "Psi," this detector could perform x-ray diffraction where LN cooled detectors could not. It too had a short snoot. The Psi succeeded were LN detectors could not, because LN dewars were too heavy to place on a goniometer arm. (Previous to the Kevex Psi, x-ray diffraction had to be performed by scintillation or gas proportional detectors and graphite monochromators). Even though the prior art short snoot detectors had fewer challenges to face with radiated and conducted heat getting in from the outside to the Si(Li) detector, these detectors had relatively high operating temperatures on the order of -80.degree. C. to -82.degree. C. Multi-stage electromechanical chiller cooled detectors produced by Kevex (supra), beginning in 1986, offered good resolution and low vibration in SEM applications. Resolution better than 149 eV FWHM at 5.9 keV at 2000 cps was guaranteed, but compressor vibration from a refrigeration system caused microphonics in the detector that vibrated SEM's and caused image resolution to deteriorate at high magnification. Under these conditions, LN-cooled detectors were recommended.
Prior art Peltier cooled detectors generally used forced air-cooling in combination with a heat sink attached to the Peltier device. Fans used to force air circulation, and compressor motors in other configurations, induced mechanical vibrations into the detectors that used them, and long snoots would amplify any vibrations that were present at the mounting base. These vibrations are enough to degrade SEM performance to the point where results are unacceptable.
The prior art of Peltier cooling of lithium-drifted silicon detectors made use of a slightly warmer heat shield to cut down on the radiated heat absorbed by the detector and its connection to the cooling device. The technique is conventional. Madden, et al., describe a method of attaching a JFET and Si(Li) detector to a coldest stage of a thermoelectric module and a heat shield that surrounds the detector to a second coldest stage of the thermoelectric module. (Madden, et al.,supra.)
SUMMARY OF THE PRESENT INVENTION
It is therefore an object of the present invention to provide a detector that eliminates the risks and costs associated with using liquid nitrogen and with filling liquid nitrogen dewars.
It is a further object of the present invention to provide a detector that reduces maintenance costs over liquid nitrogen systems.
It is a further object of the present invention to provide a detector that eliminates the microphonics associated with a long detector snoot.
It is a further object of the present invention to provide a detector that provides a more convenient design.
It is a further object of the present invention to provide a detector that eliminates the breaking of the Peltier cooling stack by shear forces resulting from twisting or levering the cold finger.
It is a further object of the present invention to provide a detector that does not affect the quality of an electron microscope's performance.
It is a further object of the present invention to provide a detector that enables superior peak resolution.
Briefly, a preferred embodiment of the present invention comprises a copper cold-finger having a lithium-drifted silicon detector and FET at one end, and a five-stage Peltier cooling stack thermally connected to the fifth stage through copper braid at the other end, an aluminum heat shield surrounding the cold-finger and thermally connected to the fourth stage of the Peltier cooling stack with a copper braid, a plurality of nylon spiders that support the cold finger within the heat shield and isolates them thermally, a stainless steel cap, a plurality of nylon spiders that support the heat shield within the cap and thermally isolates them, a heat sink in thermal communication with the hot end of the cooling stack, a remote water cooling system piped to the heat sink and mechanically isolated from vibrating the silicon detector, an ion pump to create a high-quality vacuum around the silicon detector, a motor drive to adjust the reach of the silicon detector assembly, a heat sensor, and a tip proximity sensor.
An advantage of the present invention is that liquid nitrogen is eliminated as are the risks associated with LN. Detectors incorporating the present invention are safer to operate.
Another advantage of the present invention is that no vibration is introduced that will interfere with an observation of a sample under analysis.
Another advantage of the present invention is that a preferred embodiment weighs only fifteen pounds and does not affect the quality of an electron microscope's performance.
Another advantage of the present invention is that maintenance and insurance costs are reduced. Another advantage of the present invention is that Peltier cooling stack breaking is reduced or eliminated.
Another advantage of the present invention is that resolution and performance are retained following extended temperature cycling.
Another advantage of the present invention is that superior peak resolution is demonstrated.
Another advantage of the present invention is that detector bias is immediately shut-off during periods the detector is not at its operating temperature.
Another advantage of the present invention is that a preferred embodiment may be cycled from operating temperature to room temperature at least once per week, with power off being 48 hours or longer without adverse effects. Such temperature cyclings have shown to cause no measurable degradation of resolution or light element sensitivity.
Another advantage of the present invention is that a preferred embodiment of a detector may be made portable for field use.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.